Packet headers contain protocol information that enables network elements, such as switches, routers, and host computers, to process and steer packets to their destinations. Many network functions involve changing values of header fields, as well as adding and/or removing packet headers. For example, routers in Internet Protocol (IP) networks frequently remap IP addresses of packets for purposes of network address translation (NAT). As another example, routers in a label-switched network, such as Multi-Protocol Label Switching (MPLS) routers, push and pop labels (which are a form of headers) onto and off packets that they transmit.
“Packet steering” generally includes determining the required destination of a packet and forwarding the packet to this destination. U.S. Patent Application Publication 2013/0114599, whose disclosure is incorporated herein by reference, describes a method for steering packets by a network interface controller (NIC). The method includes receiving a packet and determining parameters to be used in steering the packet to a specific destination, in one or more initial steering stages, based on one or more packet-specific attributes. The method further includes determining an identity of the specific destination of the packet in one or more subsequent steering stages, governed by the parameters determined in the one or more initial stages and one or more packet-specific attributes, and forwarding the packet to the determined specific destination. The multi-stage steering process includes a plurality of stages in which a table lookup is performed based on packet specific information, e.g., address information in the packet.